A wearer may be prescribed a positive or negative optical power correction. The ophthalmic prescription can include a power and/or an astigmatism prescription. For presbyopic wearers, the value of the power correction is different for far vision and near vision, due to the difficulties of accommodation in near vision. The prescription thus comprises a far-vision power value and an addition representing the power increment between far vision and near vision. The addition is qualified as prescribed addition. Ophthalmic lenses suitable for presbyopic wearers are multifocal lenses, the most suitable being progressive multifocal lenses.
The ophthalmic prescription can include an astigmatism prescription. Such a prescription is produced by the ophthalmologist in the form of a pair formed by an axis value (in degrees) and an amplitude value (in diopters). The amplitude value represents the difference between minimal and maximal power in a given direction which enables to correct the visual defect of a wearer. According to the chosen convention, the axis represents the orientation of one of two powers with relation to a reference axis and in the sense of rotation chosen. Usually, the TABO convention is used. In this convention, the reference axis is horizontal and the sense of rotation is anticlockwise for each eye, when looking to the wearer. An axis value of +45° therefore represents an axis oriented obliquely, which when looking to the wearer, extends from the quadrant located up on the right to the quadrant located down on the left. Such an astigmatism prescription is measured on the wearer looking in far vision. The term <<astigmatism>> is used to designate the pair (amplitude, angle); despite this use not being strictly correct, this term is also used to refer to the amplitude of the astigmatism. The person skilled in the art can understand from the context which meaning is to be considered. It is also known for the person skilled in the art that the prescribed power and astigmatism of a wearer are usually called sphere SPH, cylinder CYL and axis.
To obtain a progressive ophthalmic lens corresponding to a wearer prescription, a semi-finished ophthalmic lens blank can be provided by a lens manufacturer. Generally, a semi-finished ophthalmic lens blank comprises a first surface corresponding to an optical reference surface, for example a progressive surface in the case of traditional progressive addition lenses, and a second unfinished surface. A semi-finished lens blank having suitable optical characteristics is selected based on the wearer prescription and the unfinished surface is machined and polished by a prescription laboratory so as to obtain a lens complying with the prescription. The semi-finished lens blank may be produced by molding or by digital surfacing. The unfinished surface can be machined by digital surfacing.
A product line is defined as a family of lenses having common characteristics, i.e. common optical performances. For each product line, ranges of prescriptions are defined. A given range of prescriptions may include data relating to the ametropia of the wearer and/or data relating to the eye of the wearer. A set of semi-finished ophthalmic lens blanks are thus defined, each lens blank being adapted for manufacturing ophthalmic lenses within a given range of prescription. For instance, a product line having a given progressive design may be defined with 5 different base values, 12 different addition values and 2 different designs respectively for right and left eye, i.e. 120 reference surfaces to define in the set of semi-finished ophthalmic lens blanks. A prescription laboratory will refer to a selection chart to determine which semi-finished ophthalmic lens blank of the set to use for a given prescription.
WO-A-2009/065965 discloses a method for selecting a semi-finished lens blank according to a given spectacle frame.
A progressive ophthalmic lens may also be obtained by directly machining both surfaces using digital surfacing equipments. A raw lens blank is provided; a first aspheric surface is machined and a second aspheric surface is machined, the second aspheric surface being determined by calculation based on data relative to the first aspheric surface and on data relative to the wearer; such calculation may be an optical optimization. An ophthalmic lens complying with the prescription is thus obtained.
Whatever technology is used—starting from a semi-finished lens blank or machining both surfaces by digital surfacing—accurate positioning of the lens on the digital surfacing machine is required to ensure that the optical characteristics of the lens are fulfilled.
WO-A-2010/072749 discloses a method of manufacturing an ophthalmic lens by digital surfacing and identifies the issue of accurate positioning of the lens member on a blocker.